As means for solving the problem of an abnormal increase in the amount of the waste plastic materials due to an increased use of plastic materials in recent years, attention has been given to biodegradable plastic materials that undergo the decay by the action of enzymes which are released out of the bodies of bacteria and Eumycetes. Among these biodegradable plastic materials, the polylactic acid is drawing attention as an aliphatic polyester that is easily available being mass-produced on an industrial scale and that is environmentally friendly. Therefore, its use in various forms has been proposed in a wide range of fields.
The polylactic acid (PLA) is a resin made from such starting cereal starches as corns, and is a polymer obtained by the direct polycondensation of the lactide acid fermented product of starch or an L-lactic acid as a monomer, or is a polymer obtained by the ring-opening polymerization of a lactide which is a dimer of lactide. The polymer is also drawing attention as a resin of the type of a biologically completely recycling system since it can be decomposed into water and carbonic acid gas by the microorganisms present in the natural world.
As a recycling system of the polylactic acid in recent years, the greatest attention has been paid to a chemical recycling method which is capable of decomposing the polylactic acid and reusing it. This method comprises depolymerizing the polylactic acid by the heating in the presence of a depolymerization catalyst, and subjecting the obtained lactide to the ring-opening polymerization again to reuse it as the polylactic acid.
Patent documents 1 and 2 are proposing apparatuses for recovering the lactide from the polylactic acid that is applied to the chemical recycling. According to the apparatuses proposed by these patent documents, the polylactic acid, the depolymerization catalyst and the carrier resin are thrown into a biaxial extruder and are melt-kneaded therein. The melt-kneaded product is then conveyed by a screw in the biaxial extruder into a vent chamber (vent zone) where the lactide formed by the depolymerization of the polylactic acid is gasified, separated from other components and is recovered. Namely, the lactide of a low molecular weight (which is 144) formed by the depolymerization of the polylactic acid has a boiling point of as high as 255° C. under the standard atmospheric pressure. Therefore, upon feeding a melt-kneaded product that contains the polylactic acid and the depolymerization catalyst into the vent chamber maintained under a reduced pressure, the lactide that is formed can be recovered in a gasified form.
There is no problem if the lactide is recovered by using the above-mentioned recovering apparatuses on a laboratory scale. A problem, however, arouses if it is attempted to carry out the method on an industrial scale by throwing the polylactic acid in large amounts.
In the above method, for example, the depolymerization catalyst is mixed and, besides, the depolymerization is executed in an extruder accompanied, therefore, by a difficulty in controlling the temperature in the extruder and of ten causing the depolymerization at high temperatures. As a result, the racemization is accelerated, and the purity of the obtained lactide decreases. For example, if it is attempted to recover the L-lactide, there take place an optical isomeric transition from the L-lactide into the meso-lactide and, further, into the D-lactide due to the racemization that continues, and the purity of the desired L-lactide decreases.
In the extruder, furthermore, the carrier resin is moving while being melted and compressed, and the molten polylactic acid having a small melt viscosity and the depolymerization catalyst are conveyed by the carrier resin. Here, it has been learned by the study conducted by the present inventors that when the molten and compressed carrier resin is introduced into the vent chamber in which the pressure has been reduced, the carrier resin and the depolymerized lactide undergo the expansion since the pressure is reduced, and the carrier resin turns into a resin mass and floats on the screw conveyer passage. If the carrier resin grows into a large resin mass, the molten mixture is covered with the resin mass whereby the lactide is prevented from volatilizing. The resin mass, further, clogs the flow passage of the gaseous lactide formed by the depolymerization of the polylactic acid, and causes a great decrease in the efficiency for recovering the lactide. Moreover, the resin mass scatters and mixes into the lactide that is trapped from the vent chamber causing, therefore, serious problems.
The state where the lactide is allowed to volatilize little or is prevented from volatilizing due to the mass of the carrier resin is, usually, called “vent-up”.